1. Field of the invention
This invention relates to a wavelength tunable laser device.
2Related Background Art
A block diagram of the representative fundamental configuration of a conventional wavelength tunable laser device is shown in FIG. 1. A resonator 100 is composed of two resonator mirrors 101, 104, and a laser medium 102 and wavelength selection means 103 put therebetween. The laser medium 102 is excited by excitation means 110, and laser resonance is produced by the two resonator mirrors 101, 104. The laser resonance wavelength is determined by the wavelength selection means 103, and is outputted to the external via an optical output port 120.
FIG. 2 shows an example of an actual configuration corresponding to the block diagram of FIG. 1. The excitation means 110 is constituted as follows. A laser diode 111, a collimating leans 112, an anamorphic prism pair 113, a polarization beam splitter 114, and a focusing lens 115 are provided in order on the optical axis of the laser diode 111. Further, a laser diode 116, a collimating lens 117 and an anamorphic prism pair 118 are provided in order on the optical axis of the laser diode 116 perpendicular to the optical axis of the previously described laser diode 111. In addition, the previously described polarization beam splitter 114 is provided on the optical axis of the laser diode 116.
A laser beam from the excitation means 110, i.e., a laser beam from the focusing lens 115 is incident to a laser medium 201 having a resonator mirror 201a on its incident plane, and is reflected on a concave mirror 202. Thereafter, the laser beam thus reflected is passed through a prism 203, and is incident to a plane mirror 204 of an output port doubling as a resonator mirror.
As shown, wavelength selection means is arranged on an optical path between the resonator mirror 201a and the plane mirror 204 serving as a pair of laser resonator mirrors. In order to vary the wavelength of an output light in such a device, the prism 203 was rotated, or the plane mirror 204 of the output port doubling as a resonator mirror was rotated.
However, with such a conventional device, it is difficult to generate laser resonance because the wavelength selection means is provided within the resonator. Moreover, there are instances where when the gain of the laser medium is small, or the excitation energy is small, it is difficult to perform laser oscillation.
Further, the prism is rotated to change the wavelength of an output light, but when the rotation angle of the prism 203 is over a predetermined range, oscillation in the device stopped. Accordingly, under the necessity of operating in a near part of the longest or the shortest wavelength in the variable range, there were a great obstacle in conducting an experiment, etc. by using such a laser light source.
On the other hand, there is requested to allow the resonator mirror to have a constant transmission over a broad wavelength band in order to obtain an optical output through the resonant mirror.
However, realization of such a special mirror is not so easy to make. FIGS. 3A and 3B show actual spectral output characteristics of laser in the case where mirrors A and B are used as the plane mirror 204 of the output port doubling as a resonator mirror (in this instance, the concave mirror 202 in FIG. 2 is assumed to be a broad band total reflection mirror). FIG. 3A shows the case where the mirror A is used, and FIG. 3B shows the case where the mirror B is used.
The mirror A is caused to have a flat spectral transmission characteristic. By the use of this mirror A, it is possible to continuously change the output wavelength, e.g., over the width of 65 nm ranging from 825 nm to 890 nm. However, with this mirror A, it is impossible to allow a laser output over a broad band. On the other hand, the mirror B is made to have a small transmission characteristic over a band as broad as possible. By the use of this mirror B, it is possible to obtain an output wavelength, e.g., over the width of 92 nm ranging from 820 nm to 912 nm. However, laser oscillation was occured only at the wavelengths indicated by the points on the graph. For this reason, it was difficult to continuously change the resonance wavelength. It is considered that the transmission characteristic of the mirror B is not so flat and the transmission factor of the mirror B is too large at the wavelength where no laser oscillation is occured.
As stated above, it is ordinarily difficult to make up a mirror having a constant transmission with keeping the transmission factor small over a broad band. If an attempt is made to allow the transmission to be a constant value, there results a narrow band. In contrast, if an attempt is made to realize a broad band, the transmission fails to be constant. As a result, with conventional laser, there was the problem that there cannot help being an output characteristic greatly dependent upon the characteristic of the resonator mirror. In addition, in the output characteristics shown in FIGS. 3A and 3B, the spectral width at the time of laser oscillation at respective wavelengths was a relatively broad of about 4 nm (FWHM: Full Width at Half Maximum).